The present invention relates to a digital cordless telephone system, such as a PHS (personal handy phone system) in Japan and a DECT (digital European cordless telecommunications), for communicating between a base station and a terminal station by transmitting and receiving digital data and, more particularly to a base station of a digital cordless telephone system.
FIG. 1 of the accompanying drawings shows a digital cordless telephone system in block form As shown in FIG. 1, a base station 1 is connected to a PSTN (public switched telephone network) and includes an antenna la for making a communication between it and a terminal station. There are prepared a plurality of, e.g., three terminal stations 2, 3, 4 which are capable of communicating with the base station 1. Antennas 2a, 3a, 4a are attached to the terminal stations 2, 3, 4 and digital data is transmitted and received between the antenna 1a of the base station 1 and the antennas 2a, 3a, 4a of the terminal stations 2, 3, 4 via radio waves.
When each of the terminal stations 2, 3, 4 communicates with the base station 1 (or the called party connected to the base station 1 via the PSTN), the terminal station transmits a connection control signal of a predetermined format to the base station 1 and communicates with the base station 1 by transmitting audio data converted in the form of time-division digital data.
The digital cordless telephone system uses a TDD (time division duplex) system in which a transmission frequency and a reception frequency are the same. Also, the digital cordless telephone system uses a TDMA (time division multiple access) system for transmitting a plurality of communication signals at the same frequency during different periods.
Specifically, in the case of the TDD system, as shown in FIG. 2A, one channel (frequency) is divided into a transmission slot T and a reception slot R from a time standpoint and these transmission and reception slots T, R are repeated alternately. Moreover, a guard time G is provided between these slots T and R. A duration of each of the slots T and R is selected to be 1 millisecond and a duration of the guard time G is selected to be several 10s of microseconds In the portable telephone (terminal station), the transmission slot T communicates with the base station (master station) and the reception slot R receives audio data from the base station.
In the case of the TDMA system, as shown in FIG. 2B, the transmission slot T and the reception slot R disposed within one channel are used only one time during several periods in order to communicate the single terminal station and the base station. Other transmission slots T and reception slots R disposed within the same channel are used in order to communicate between other terminal stations and the base station. Thus, digital data is multiplexed in order to allow communication among a plurality of communication apparatus on one channel.
Since audio data is multiplexed as described above, the frequency band prepared for the cordless telephone can be used efficiently. When such communication is carried out, a control signal is transmitted from the base station to each terminal station by transmitting the control signal on a slot arrangement at a predetermined period on a different channel (control channel) from the transmission and reception channels of the communication signal. Specifically, as shown in FIG. 3A, the base station repeatedly transmits a control signal with slot of 625 microseconds at the period of 125 milliseconds. The reason that the control signal is transmitted at the period of 125 milliseconds is that the control signal is transmitted 8 times per second according to the standards of this kind of digital cordless telephone. When the control signal is transmitted 8 times per second at an equal interval the period becomes 125 milliseconds.
Each terminal station can discriminate control information indicative of assignment of slots used in transmission and reception by receiving the transmitted control signal. A communication between the base station and the terminal station becomes possible by use of the slots designated by the control information. When each terminal station is called by an incoming call or incoming call extension, the base station transmits control information indicative of the terminal station called by the incoming call or incoming extension by the control signal. Accordingly, each terminal station has to receive the control signal constantly even when placed in the reception standby mode. In this case, however, from a standpoint of power consumption in the reception standby mode, it is unnecessary for the terminal station to receive all control signals transmitted from the base station at the interval of 125 milliseconds when placed in the reception standby mode. Accordingly, in actual practice, each terminal station intermittently receives the control signal once per several seconds.
If a plurality of systems are located in the areas close to each other, there is then the possibility that control signals transmitted from the respective base stations will interfere with each other, thereby making it impossible for the terminal stations of the respective system to receive the control signals.
Specifically, let it be assumed that two base stations of the cordless telephone systems where the transmitted states of the control signals are the same are located at the places comparatively close to each other (e.g., within a range of 100 m). Also let it be assumed that the transmission timings of the control signals from the two base stations are almost agreed with each other as shown in FIGS. 3A and 3B. At that time, a terminal station located in the area being capable of receiving the control signals transmitted from the two base stations are urged to receive the control signals from the two base stations at the same time. As a result, this terminal station cannot accurately discriminate data contained in the control signal of each slot. If the terminal station becomes unable to discriminate the control signal, then such terminal station becomes unable to set a communication between it and the base station. Thus, the digital cordless telephone system cannot be used as the cordless telephone.
In actual practice, even though the transmission timings of the control signals of one slot of 625 microseconds are not completely synchronous, if the control signal is transmitted from one base station at a timing overlapping a little of the leading or trailing edge of the other control signal (control signal shown in FIG. 3A) as shown by phantoms in FIG. 3B, there is then the large possibility that the control signal will not be received by the terminal station.
Assuming now that a frequency difference of clocks for determining timings at which the two base stations transmit control signals is 5 ppm, then the transmission timing of the control signal is shifted by 5 microseconds per second. If the transmission timings of the two control signals start overlapping at a certain time, then the control signals interfere with each other over 375 seconds because 625.times.3/5=375. As a consequence, the terminal station cannot receive an incoming call and cannot make an outgoing call during a time period of 375 seconds.